a. Statement of the Invention
The present invention relates to chemically joined, phase separated thermoplastic graft copolymers.
B. Description of the Prior Art
Polymer technology has developed to a high degree of sophistication and extensive research efforts in this direction are being undertaken to obtain improvements in polymer properties. Some of these efforts lead to polymer materials capable of competing with metals and ceramics in engineering applications. Generally, it is a requirement that these polymers be crystalline, since crystalline polymers are strong, tough, stiff and generally more resistant to solvents and chemicals than their non-crystalline counterparts.
Many poly alpha-olefins are crystalline and have excellent structural integrity; and, accordingly, have acquired increasing commercial acceptance as materials for competing with metals and ceramics. As one example, polyethylene has been regarded as one of the most important polymers among the major plastics, with its production reaching about 6 billion pounds in 1970 (1.7 billion pounds of high density linear polyethylene and 4.3 billion pounds of low density polyethylene).
Despite the widespread use of this important plastic, its use has been limited to flexible, translucent, molded articles or flexible, clear films, due to its softness. The uses of polyethylene have also been limited due to its poor adhesion to many substrates and its low heat distortion, rendering it unsuitable for many high temperature applications.
Attempts by prior art workers to combine the properties of polyolefins and other polymers by either chemical or mechanical means generally has resulted in a sacrifice of many of the beneficial properties of both the polyolefin and the additional polymer. For example, graft copolymers of polyethylene and polypropylene have been prepared only with difficulty due to the inertness these polymers have with many other polymerizable monomers and polymers. The resultant graft copolymer generally has been a mixture which also contains free homopolymers.
Polyblends of a polyolefin with another polymer prepared by blending quantities of the two polymers together by mechanical means have been generally unsuitable for many applications due to their adverse solubility or extractability properties when used with various solvent systems, particularly when containing a rubbery, amorphous component.
The above considerations recognized by those skilled in the art with respect to the incompatibility of polyolefins with other polymers find almost equal applicability in the case of other plastics such as the polyacrylates, polymethacrylates, polyvinylchlorides, etc. Thus, the incompatibility of both natural and synthetic polymers becomes increasingly apparent as more and more polymers having particularly good properties for special uses have become available, and as efforts have been made to combine pairs of these polymers for the purpose of incorporating the different, good properties of each polymer into one product. More often than not, these efforts have been unsuccessful because the resulting blends have exhibited an instability, and in many cases the desirable properties of the new polymers were completely lost. As a specific example, polyethylene is incompatible with polystyrene and a blend of the two has poorer physical properties than either of the homopolymers. These failures were at first attributed to inadequate mixing procedures, but eventually it was concluded that the failures were due simply to the inherent incompatibilities. Although it is now believed that this is a correct explanation, the general nature of such incompatibility has remained somewhat unclear, even to the present. Polarity seems to be a factor, i.e., two polar polymers are apt to be more compatible than a polar polymer and a non-polar polymer. Also, the two polymers must be structurally and compositionally somewhat similar if they are to be compatible. Still further, a particular pair of polymers may be compatible only within a certain range of relative proportions of the two polymers; outside that range they are incompatible.
Despite the general acceptance of the fact of incompatibility of polymer pairs, there is much interest in devising means whereby the advantageous properties of combinations of polymers may be combined into one product.
One way in which this objective has been sought involves the preparation of block or graft copolymers. In this way, two different polymeric segments, normally incompatible with one another, are joined together chemically to give a sort of forced compatibility. In such a copolymer, each polymer segment continues to manifest its independent polymer properties. Thus, the block or graft copolymer in many instances possesses a combination of properties not normally found in a homopolymer or a random copolymer.
Recently, U.S. Pat. No. 3,235,626 to Waack, assigned to Dow Chemical Company, described a method for preparing graft copolymers of controlled branch configuration. It is described that the graft copolymers are prepared by first preparing a prepolymer by reacting a vinyl metal compound with an olefinic monomer to obtain a vinyl terminated prepolymer. After protonation and catalyst removal, the prepolymer is dissolved in an inert solvent with a polymerization catalyst and is thereafter reacted with either a different polymer having a reactive vinyl group or a different vinyl monomer under free-radical conditions.
The current limitations on the preparation of these copolymers are mechanistic. Thus, there is no means for controlling the spacing of the sidechains along the backbone chain and the possibility of the sidechains having irregular sizes. Due to the mechanistic limitations of the prior art methods, i.e., the use of an alpha-olefin terminated prepolymer with acrylonitrile or an acrylate monomer, complicated mixtures of free homopolymers result.
In view of the above considerations, it would be highly desirable to devise a means for preparing graft copolymers wherein the production of complicated mixtures of free homopolymers is minimized and the beneficial properties of the sidechain and backbone polymer are combined in one product.
Moreover, it is recognized and documented in the literature, such as R. Waack et al, Polymer, Vol. 2, pp. 365-366 (1961), and R. Waack et al, J. Org. Chem., Vol. 32, pp. 3395-3399 (1967), that vinyl lithium is one of the slowest anionic polymerization initiators. The slow initiator characteristic of vinyl lithium when used to polymerize styrene produces a polymer having a broad molecular weight distribution due to the ratio of the overall rate of propagation of the styryl anion to that of the vinyl lithium initiation. In other words, the molecular weight distribution of the polymer produced will be determined by the effective reactivity of the initiator compared with that of the propagating anionic polymer species, i.e., vinyl lithium initiator reactivity compared to the styryl anion. Accordingly, following the practice of U.S. Pat. No. 3,235,626, a graft copolymer having sidechains of uniform molecular weight cannot be prepared.
U.S. Pat. Nos. 3,390,206 and 3,514,500 describe processes for terminating free-radical and ionic polymerized polymers with functional groups which are described as capable of copolymerizing with polymerizable monomers. The functionally terminated prepolymers described by these patentees also would be expected to have a broad molecular weight distribution and, therefore, would not be capable of producing a chemically joined, phase separated thermoplastic graft copolymer.